Various publications, including patents, published applications, and scholarly or technical articles are cited throughout the specification. Each of the cited publications is incorporated by reference herein, in its entirety.
As a complex and sensitive organ of the body, the eye can experience numerous diseases and other deleterious conditions that affect its ability to function normally. Many such conditions can be found in the interior and most particularly at the rear of the eye, and affect the optic nerve and the retina, seven layers of alternating cells and processes that convert a light signal into a neural signal. Diseases and degenerative conditions of the optic nerve and retina are the leading causes of blindness throughout the world.
Cataracts are another pathology of the eye, and are characterized by a progressive opacification of the lens. The lens of the eye, located between the iris and the vitreous body, functions to focus light onto the retina. The loss of optical clarity of the lens disrupts its ability to focus light, and results in visual impairment and blindness. Cataracts are a leading cause of vision loss worldwide, and affect all demographics. Treatment of cataracts generally constitutes surgical removal of the affected lens, and replacement with an intraocular lens.
Cataracts are classified into one of three categories, based on their clinical appearance: Nuclear, which is characterized by the hardening and discoloration of the inner-most lens fibers; Cortical, which is characterized by opacification of fibers on the outside of the lens; and Subcapsular, which are opacities found primarily in the posterior portion of the lens, typically under the posterior capsule. The etiology of cataracts is not well understood, although many risk factors have been identified. Such risk factors include advanced age, genetics, gender, obesity, diabetes, and other assorted medical problems, as well as from environmental factors such as UV light, and oxygen. (Asbell P. A. et al. “Age-related cataract” Lancet. 2005, 365:599-609.)
Age-related cataracts result from gradual opacification of the crystalline lens of the eye. It is believed that once begun, cataract development proceeds via one or more common pathways that culminate in damage to lens fibers. This condition progresses slowly and occurs predominantly in the elderly. Alternatively, cataract may form because of surgical, radiation or drug treatment of a patient, e.g., after surgery of an eye to repair retinal damage (vitrectomy) or to reduce elevated intraocular pressure; x-irradiation of a tumor; or steroid drug treatment.
Increasing evidence indicates that an oxygen gradient is maintained within the vitreous humor of the eye, with the lowest amount of oxygen nearest to the lens, and the highest amount nearest to the retina. (Buerk, D. G. et al. “O2 gradients and countercurrent exchange in the cat vitreous humor near retinal arterioles and venules” Invest. Ophthalmol. Vis. Sci. 1993, 41:3061-3073; and Barbazetto I. A. et al. “Oxygen tension in the rabbit lens and vitreous before and after vitrectomy” Exp. Eye Res. 2004, 78:917-924). Disruptions in this oxygen gradient have been proposed to contribute to cataract formation. (Holekamp, N. M. et al. “Vitrectomy surgery increases oxygen exposure to the lens: A possible mechanism for nuclear cataract formation.” Am. J. Ophthalmol. 2005, 139:302-310).
The mechanisms by which the oxygen gradient in the vitreous humor is maintained are not well understood, although the vitreous gel apparently plays a crucial role. (Holekamp, N. M. 2005) It stands to reason that removal of the vitreous gel, as occurs in vitrectomy procedures, results in the disruption of this natural oxygen gradient.
Vitrectomies are typically indicated for, among other things, repairs to the retina, as in the case of the retinal pathologies listed above, retinal tears, or retinal detachment, for removal of blood from the vitreous humor, for the repair of macular holes, for the repair of eye trauma or for the removal of foreign objects, and for clearing vitreous infections. The vitrectomy procedure involves removal of the vitreous gel of the eye, and replacement of the vitreous gel with a balanced salt solution, air, a gas such as SF6, a fluorocarbon, or a silicon oil to maintain eye pressure and shape. Following the vitrectomy, the body gradually replenishes the vitreous gel.
The vitrectomy disrupts the oxygen gradient in at least two ways. First, the surgical incisions allow oxygenated air to penetrate the vitreous cavity, and the air perfusion is further facilitated by the insertion of the surgical instruments into the eye. Second, the solution used to temporarily replace the vitreous gel contains oxygen levels much higher than the vitreous gel, and the oxygen is evenly distributed throughout the solution. (Barbazetto, I. A. 2004). As a consequence of the disruption of the oxygen gradient, the lens is exposed to levels of oxygen determined to be 2-3 times higher than normal. (Barbazetto I. A., 2004) Moreover such high levels of oxygen persist in the vitreous cavity at least 10 months following surgery. (Holekamp, N. M. 2005).
Exposure of the lens to oxygen plays a role in the formation of cataracts. Furthermore, it has been demonstrated that cataracts frequently form in patients following a vitrectomy procedure, especially if the patient is over 50. (Chung, C. P., et al. “Cataract formation after pars plana vitrectomy” Kaohsiung J. Med. Sci. 2001, 17:84-89; and Hsuan, J. D., et al. “Posterior subcapsular and nuclear cataract after vitrectomy” J. Cataract Refract. Surg. 2001, 27:437-444). Thus, a significant advancement in vitrectomy procedures would be to have available a pharmaceutical composition to administer to the patient before, during, or after the vitrectomy in order to minimize the risk of developing a cataract following the surgery by diminishing or preventing oxidative stress caused by the disruption of the natural oxygen gradient.
In addition to its role in the development of cataracts, oxidative stress has been implicated in the development or acceleration of numerous ocular diseases or disorders, including AMD and various other retinopathies. (see, e.g., Ambati et al., 2003, Survey of Ophthalmology 48: 257-293; Berra et al., 2002, Arch. Gerontol. Geriatrics 34: 371-377), as well as uveitis (e.g., Zamir et al., 1999, Free Rad. Biol. Med. 27: 7-15), glaucoma (e.g., Babizhayev & Bunin, 2002, Curr. Op. Ophthalmol. 13: 61-67), corneal and conjuctival inflammations, various corneal dystrophies, post-surgical or UV-associated corneal damage (e.g., Cejkova et al., 2001, Histol. Histopathol. 16: 523-533; Kasetsuwan et al., 1999, Arch. Ophthalmol. 117: 649-652), and presbyopia (Moffat et al., 1999, Exp. Eye Res. 69: 663-669). For this reason, agents with anti-oxidative properties have been investigated as potential therapeutic agents for the treatment of such disorders. Many investigations have focused on the biochemical pathways that generate reducing power in cells, for example, glutathione synthesis and cycling. Enzymes, such as superoxide dismutase, that reduce activated oxygen species have also been studied to determine whether they diminish cellular oxidative stress. Compounds for inhibiting lipid oxidation in cell membranes by direct radical scavenging have also been considered to be promising therapeutic interventions.
Nitroxides are stable free radicals that are reducible to their corresponding hydroxylamines. These compounds are of interest because of their radical scavenging properties, mimicking the activity of superoxide dismutase and exerting an anti-inflammatory effect in various animal models of oxidative damage and inflammation. Nilsson et al. disclosed, in WO 88/05044, that nitroxides and their corresponding hydroxylamines are useful in prophylaxis and treatment of ischemic cell damage. Paolini et al. (U.S. Pat. No. 5,981,548) disclosed N-hydroxylpiperidine compounds and their potential general utility in the treatment of pathologies arising from oxygen radicals and as foodstuff and cosmetic additives. Hsia et al. (U.S. Pat. Nos. 6,458,758, 5,840,701, 5,824,781, 5,817,632, 5,807,831, 5,804,561, 5,767,089, 5,741,893, 5,725,839 and 5,591,710) disclosed the use of stable nitroxides and hydroxylamines (e.g., tempol and its hydroxylamine counterpart, tempol-H), in combination with a variety of biocompatible macromolecules, to alleviate free radical toxicity in blood and blood components. Hahn et al. (1998, Int. J. Radiat. Oncol. Biol. Phyics 42: 839-842; 2000, Free Rad. Biol. Med. 28: 953-958) reported on the in vivo radioprotection and effects on blood pressure of the stable free radical nitroxides and certain hydroxylamine counterparts.
In ocular disorders, Zamir et al. (1999, supra) reported that the nitroxide 4-hydroxy-2,2,6,6,-tetramethylpiperidine-1-N-oxyl (TPL or tempol) reduced the severity of retinal S-antigen-induced experimental autoimmune uveoretinitis (EAU) after systemic injection in a rat model. Reddan et al. (1993, Exp. Eye Res. 56: 543-554) reported an investigation into the use of the nitroxide tempol to protect lens epithelial cells from hydrogen peroxide damage in vitro. Mitchell et al. (U.S. Pat. No. 5,462,946) also disclosed use of nitroxides (such as tempol) to protect lens epithelial cells from oxidative damage. Though Mitchell et al. also reported that the corresponding hydroxylamine tempol-H afforded no such protection (Mitchell et al., 1991, Arch. Biochem. Biophys. 289: 62-70; Krishna et al., 1991, Cancer Research 51: 6622-6628), Zigler et al. (U.S. Pat. No. 6,001,853) reported to the contrary, disclosing that the hydroxylamine was a better anti-cataractogenic composition than the corresponding nitroxide.
Due to their comparative lack of toxicity, hydroxylamines are preferable to nitroxides as therapeutic agents. However, outside the highly reducing environment of the ocular lens (M. Lou, 2003, supra), there has been no report of the use of hydroxylamine compositions for the treatment against oxidative stress to the eye that occurs during a vitrectomy procedure. Accordingly, there remains a substantial, yet unmet, need for safe, clinically useful, treatments to prevent, repress, or slow the development of cataracts and other eye disorders that arise as the result of a vitrectomy procedure.